In virtual acoustics, it is common to simulate the early part of a
Room Impulse Response using approaches from geometrical acoustics and the late part using Feedback Delay Networks (FDNs). In
order to transition from the early to the late part, it is useful to
slowly fade-in the FDN response. We propose two methods to control the fade-in, one based on double decays and the other based
on modal beating. We use modal analysis to explain the two concepts for incorporating this fade-in behaviour entirely within the
IIR structure of a multiple input multiple output FDN. We present
design equations, which allow for placing the fade-in time at an
arbitrary point within its derived limit.